This invention relates to a new and improved freezing system suitable for the freezing of food products, and in particular, to a new and improved freezing system combining a cryogenic freezer and a mechanical refrigeration freezer.
Mechanical refrigeration freezers have been known and used for many many years. Cryogenic freezers have been known and used for more than twenty five years and two such freezers are shown in U.S. Pat. Nos. 3,832,864 and 4,403,479.
Cryogenic freezing systems provide cooling by vaporizing liquefied gases, such as carbon dioxide (C0.sub.2), nitrogen (LN.sub.2) and others. This process is capable of producing low temperatures to -320.degree. F. Typically the product to be frozen is immersed in the cryogenic liquid, or the cryogenic liquid is sprayed onto the product. Cryogenic systems are also called "Expendable Refrigerant Systems " since the recovery of the cryogenic fluid is not usually attempted.
Mechanical refrigeration systems, usually called conventional, provide cooling by evaporation, compressing and condensing various refrigerants in a closed loop system. Mechanical refrigeration systems usually produce temperatures to -40.degree. F. Two-stage and cascade systems are capable of producing low temperatures to about -120.degree. F.
Positive features of cryogenic systems include the following. Very fast cooling/freezing, resulting in better quality and minimal dehydration (weight loss) of the product, usually less than 1%. Substantially smaller and less costly equipment for the given freezing capacity. The product is enveloped in an oxygen-free atmosphere which eliminates so called "freezer burns " and usually results in better product quality. Cryogenic systems are usually more suitable for so called IQF (individually quick frozen) products. The immersion of products in liquid nitrogen produces boiling of the liquid and IQF products are easily obtained.
A negative aspect of tne cryogenic system is the usually higher cost of freezing, especially for low cost products such as fruits and vegetables. Freezing fresh meats usually requires 1 pound of LN.sub.2 or 11/2 pounds of CO.sub.2 per pound of meat. Fruits and vegetables require even more; 11/2 to 2 pounds of LN.sub.2 or CO.sub.2 per pound. The cost of CO.sub.2 or LN.sub.2 is usually 4 to 8 per pound of frozen product.
The most important feature of mechanical refrigeration is the lower cost of freezing after the initial cost of the equipment is amortized. The cost of freezing is usually 3 to 4 per pound of product, depending on the cost of electricity in a given area.
Major weaknesses of mechanical refrigeration freezers include the following. They require a substantial amount of floor space. They are very costly and require a lot of electrical power. The slower freezing results in a lower quality product. Cleaning and maintenance are costly and require a considerable amount of down time. The cooling coils need to be defrosted every three to four hours, and the necessary halt in the freezing operation interrupts continuous operation of other production lines. Weight loss (dehydration) of the product is 2 to 8% and, ir accounted in the cost of freezing, in some cases may be more expensive than cryogenic freezing. Most mechanical systems cannot produce IQF quality of product, rather the pieces freeze together and to the conveyor belt.
Freezing food and other products has become the most popular method of preservation, particularly since the discovery of ill-effects of various chemical preservatives. The food industry, in particular, has a need for freezing systems that can produde the best possible product quality and at the lowest possible initial and operating costs. Other desirable features for a better freezing system are compactness, operating flexibility and capability to produce IQF quality products.